Arrests Come as Worries Grow Over Advances by Islamic State in Neighboring Iraq

RIYADH—Authorities in Saudi Arabia arrested 88 people on suspicion of planning to carry out terrorist attacks both in the kingdom and abroad, the Ministry of Interior said on Tuesday, as worries escalated in the Gulf nation over the advance of Islamic State rebels in neighboring Iraq.

The Islamic State's declaration in June of a caliphate encompassing parts of both Iraq and Syria, following its lightning military advance across...